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The present invention relates generally to electronic devices and more particularly to a direct-current (xe2x80x9cDCxe2x80x9d) blocking capacitor integrated into the center conductor of a coaxial transmission structure.
Blocking capacitors (commonly called xe2x80x9cDC blocksxe2x80x9d) are used in a variety of applications to couple alternating current (xe2x80x9cACxe2x80x9d) of sufficient frequency across the capacitor while blocking DC current Blocking capacitors have a cutoff frequency, below which AC is not efficiently coupled across the capacitor, and a self-resonant frequency that typically limits the upper frequency of operation. Generally, a lower cutoff frequency can be achieved with a greater capacitance, and a higher self-resonant frequency can be achieved with a physically smaller capacitor.
Blocking capacitors are incorporated in electronic circuits, such as at the input of an amplifier or mixer in a series configuration, to keep DC from damaging the circuit. Incorporating a conventional DC block in a packaged microcircuit typically involves die attaching one plate of a capacitor to the microcircuit and then wire-bonding or mesh-bonding the other plate of a capacitor to another portion of the packaged microcircuit, such as a feedthru pin. This increases assembly time and occupies additional room inside the packaged microcircuit. The conventional DC block often disrupts the transmission characteristics of the circuit, so compensating for the disruption by manipulating the wire-bond or mesh-bond, or by adding tuning elements, such as poly-iron, inside the packaged microcircuit, is employed, which increases the assembly time of the microcircuit.
Coaxial DC blocks incorporate a capacitor along the electrical path of the center conductor of a coaxial structure, such as a coaxial transmission line, a coaxial connector, or a coaxial feedthru. Coaxial DC blocks can be integrated into a packaged microcircuit, an external device, such as a connector, adaptor, or bias-T, or integrated into a port of a test instrument, such as a network analyzer, spectrum analyzer, or signal generator.
FIG. 1 is a simplified cross-section of a first prior-art coaxial DC block 10. A capacitor 12 is attached to the end face a first center conductor half 14 of a coaxial structure. A bellows 16 is held in a pocket 18 of a second center conductor half 14xe2x80x2. The bellows are electrically conductive and press against the metallized plate 13 of the capacitor 12. The other plate 11 of the capacitor is soldered or otherwise electrically and mechanically coupled to the first center conductor half 14. The capacitance of the capacitor is a function of the area of the plates of the capacitor, the distance between the plates, and the dielectric constant of the material 15 between the plates.
The bellows 16 approximate the diameter of the center conductor to maintain the characteristic impedance of the coaxial structure. Both center conductor halves 14, 14xe2x80x2 must be supported to maintain contact (compression) and alignment. The gap that the bellows 16 occupies varies from assembly to assembly, and the bellows compensate for the manufacturing tolerance build-up of the other parts of the DC block by extending or compressing. The diameter of the capacitor 12 is less than or equal to the diameter of the center conductor, so if the center conductor is small the associated capacitor might have an undesirably low capacitance, resulting in a higher cutoff frequency.
The type of coaxial DC block illustrated in FIG. 1 can achieve good results if the cross-sectional area of the center conductor is sufficiently large or the intended operating frequency is limited. For example, the type of coaxial DC block illustrated in FIG. 1 can work well in an N-type coaxial connector because the center conductor is relatively large. In these N-type coaxial connectors the coaxial DC block can provide acceptable performance from about 10 MHz to about 18 GHz.
FIG. 2 is a simplified cross section of a second prior art coaxial DC block 20 that uses multiple capacitors. Two parallel-plate capacitors 22, 24 are held in a special clip 26 that mechanically supports the capacitors and electrically connects the outer plates 28, 30 of the capacitors to a first center conductor half 14. Axially resilient coaxial connections 32, 34 press against the clip 26 and the inner plates 36, 38 of the capacitors to create a solderless electrical connection with a second center conductor half 14xe2x80x2.
Unfortunately, the clip 26 and the capacitors 22, 24 extend relatively far beyond the circumference of the center conductor halves 14, 14xe2x80x2. Creating a discontinuity, such as a change in the diameter in the center conductor that disrupts the characteristic impedance, affects the transmission of high-frequency electrical signals through the coaxial DC block. These discontinuities are especially difficult to compensate for at millimeter frequencies. In addition, transmission of high-frequency signals through the compressive contacts are susceptible to shock and vibration as the parts move relative to each other. Additionally, both center conductor halves 14, 14xe2x80x2 have to be firmly secured to maintain the compressive contact of the resilient coaxial connections against the capacitors.
A DC block constructed according to the embodiments of the present invention includes a first conductor half and a second conductor half attached to plates of a capacitor extending along a longitudinal axis of the center conductor. The capacitor electrically couples alternating current between the first and second conductor halves and securely attaches the first conductor half to the second conductor half. In some embodiments, two or more capacitors are soldered to the first and second conductor halves in parallel, thus increasing the capacitance between the first and second conductor halves. In other embodiments, a radial capacitor, such as a cylindrical capacitor, is disposed within an outer conductor half.
A method according to an embodiment of the present invention fabricates a coaxial DC block from a center conductor of a coaxial structure, such as a center pin of a coaxial connector or feedthru. One or more parallel-plate capacitors are positioned in mounting slots formed in the center pin before the center pin is separated into first and second center conductor halves. This maintains alignment of the center conductor halves and length of the center pin during fabrication of the coaxial DC block.